Research is focused on studying the events involved in HIV-1 fusion, the development of fusion inhibitors directed against gp41 and the development of monoclonal antibodies with broadly neutralizing activity that are directed against gp41. The HIV-1 Envelope (Env) proteins that mediate membrane fusion represent a major target for the development of new AIDS therapies. Three classes of Env-mediated membrane fusion inhibitors have been described that specifically target the pre-hairpin intermediate conformation of gp41. Class 2 inhibitors bind to the C-terminal heptad repeat (C-HR) of gp41. The single example of a class 3 inhibitor targets the trimeric N-terminal heptad repeat (N-HR) of gp41 and has been postulated to sequestrate the N-HR of the pre-hairpin intermediate through the formation of fusion incompetent heterotrimers. We have now shown that NCCG-gp41, a class 2 inhibitor, and N36Mut(e,g), a class 3 inhibitor, synergistically inhibit Env-mediated membrane fusion for several representative HIV-1 strains (X4 and R5) in both a cell fusion assay (with membrane-bound CD4) and an Env-pseudotyped virus neutralization assay. We have also succeeded in obtaining a monoclonal Fab (Fab 3674) selected from a human non-immune phage library by panning against the chimeric construct NCCG-gp41 (that comprises an exposed coiled-coil trimer of gp41 N-helices fused in helical phase onto the minimal thermostable ectodomain of gp41) that effectively neutralizes diverse laboratory-adapted B-strains of HIV-1 and primary isolates of subtypes A, B and C in an Env-pseudotyped virus neutralization assay, albeit with reduced potency (25x) compared to 2F5 and 4E10. Alanine scanning mutagenesis maps a novel epitope to a shallow groove on the N-helices of gp41 that is exposed between two C-helices in the fusogenic six-helix bundle conformation of gp41. We have shown that bivalent Fab 3674 and the C34 peptide (a potent fusion inhibitor derived from the C-helix of gp41) act at similar stages of the fusion reaction and to neutralize HIV-1 synergistically, providing additional evidence that the epitope of Fab 3674 is new and distinct from the binding site of C34.